Fuel cells are electrochemical devices that convert chemical energy directly to electrical energy. The by-products are typically water and heat. The conversion process is quiet and efficient, making fuel cells attractive for a wide range of applications. Individual fuel cells typically generate a very low output voltage, such as below 1V. To increase the voltage to usable levels, the cells are connected in series and are known as fuel cell stacks. Individual fuel cells can be used for small power packs while larger systems may require a number of stacks connected in such a way so as to produce acceptable voltage levels.
The most commonly used fuel cell technology is the Polymer Electrolyte Membrane (PEM) fuel cell. PEM fuel cells have high power density, high efficiency, and low start-up times making them widely suitable. Single cells may be used to power portable electronics while PEM fuel cell stacks are used as backup power systems. PEM fuel cells are also the technology of choice for automotive manufacturers developing hydrogen fuel cell vehicles.
PEM fuel cells operate below 100° C. and require humidification of their membranes in order to enable proton conduction for proper operation. The need for humidification combined with the internal generation of water gives rise to a number of operational problems. The most common of these include drying and flooding. Other fault conditions can include fuel starvation due to insufficient gas supply rates from the anode or cathode mass flow control systems, or hydrogen crossing over the membrane. The onset of these fault conditions can cause severe loss in performance and in extreme cases cause permanent damage to the internal components of the fuel cell. Thus, it is of great importance to monitor the state of health of fuel cells in order to diagnose possible fault conditions and enable a master control system to take appropriate action to mitigate the fault mechanism. By continuously monitoring the state of health, the system can also optimize operating conditions such as the relative humidity (RH), fuel supply rates or temperature to maximise performance.
When fuel cells are in an inactive state (i.e. not powering a load), techniques for diagnosing the inner phenomena occurring in the fuel cell include polarization curve analysis, where voltage is recorded as a function of current or the other way around, and cyclic voltammetry, where the electrocatalytic surface area of catalyst layers can be determined by sweeping an applied voltage between two set-points and recording the current. These techniques do not work when a fuel cell system is operational and powering a load.
An industry standard for monitoring a fuel cell system while it is operational is by performing Electrochemical Impedance Spectroscopy (EIS). EIS makes use of an expensive Frequency Response Analyzer (FRA) that introduces a voltage or current waveform with a set frequency and amplitude and superimposes the waveform on the DC loading point. The response is then recorded and the impedance calculated. By varying the frequency of the waveform, a plot of impedance versus frequency, called a Nyquist plot, is then produced to represent the impedance trajectory for a range of frequencies by plotting it on the real and imaginary axis. The Nyquist plot gives valuable information on the internal mechanisms of the fuel cell and is a powerful tool for condition monitoring and determining state of health. Existing EIS systems take up to several minutes to produce a Nyquist plot as each frequency harmonic is individually introduced to the fuel cell to limit disturbance.
Fuel cells exhibit extreme non-linear behaviour under fault conditions such as flooding, drying or fuel starvation. It is thus extremely difficult to achieve stable measurements of the impedance for the full frequency range of interest. For the measurements to be acceptable, conditions such as linearity, stability and causality must exist during the measurement period. Because EIS systems take up to several minutes to perform measurements, they are not suitable for diagnosing many fault conditions in fuel cells.
In other kinds of electricity-producing cells, such as electrochemical cells, monitoring the state of health through determining impedance measurements is also important, and current techniques such as EIS are generally time-consuming and may not be suitable during non-linearities particular to those cells.
The technology described in this application seeks to address these problems, at least to some extent.
In this specification, the term “electricity-producing cell” has a wide meaning and includes cells such as fuel cells and electrochemical cells.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.